This invention relates to photogrammetric apparatus and, more particularly, to a platen for forming reseau marks on film.
In the photographing of terrain or the like with a mapping camera or the like, a common practice is to generate a set of discrete punctiform images or reseau marks on the photographic format or film which are uniformly distributed throughout the film while the optical image of a scene is formed by the camera. Such reseau marks are used to facilitate measurements through which appropriate corrections can be made to remove the effects of film deformation and thereby improve accuracies of aerotriangulation.
The typical camera which provides for the formation of such reseau marks utilizes a glass reseau plate provided with reseau marks and which is placed in front of the focal plane in the camera. However, the glass reseau plate camera has never achieved widespread acceptance among potential users because of its many limitations and drawbacks. For instance, the glass reseau plate produces marks which are quite conspicuous and distracting on the developed film inhibiting photo interpretation. In addition, the glass reseau plate is subject to gradual deterioration by scratching and abrasion, generates discharges of static electricity, constitutes a compromising element of the optical system, and is potentially subject to breaking or misalignment. Furthermore, such glass reseau plates are generally part of the camera cone, limiting the type of camera in which it may be utilized. Another specific drawback of the glass reseau plate arises with film images which contain either overexposed or underexposed areas. Since the glass reseau plate system is a passive system depending on the illumination of the object photographed, measurement difficulties arise when measurements are made through such overexposed or underexposed areas, and, since the glass reseau plate always appears in the focal plane, reseau marks are formed on the film whether they are desired or not.
There has been in the past some limited utilization of a platen for forming reseau marks on a photographic format, but such reseau platens have not been commercially available, their main use being in a few very expensive military mapping cameras. However, even such present-day reseau platens have objectionable characteristics, one of which was a deformation of the platen upon the application of the film-flattening vacuum. Vacuum-induced deviations from flatness of as much as 30 .mu.m have been found to be characteristic of platens in certain well-known commercial mapping cameras. Such deformation can significantly compromise accuracies of aerial triangulation and do damage to the integrity of the stereo model. Basically, such vacuum-induced deformation is brought about with present-day platens by the use of a vacuum plenum behind the film format.